Tracking sensor systems e.g. a radar system or subsystems such as Raytheon's Mariner 2200 and AN/SPS-73 radar system are used to monitor marine traffic. These systems meet the specifications of the International Maritime Organization for automatic radar plotting aids and thus can be used to track other vessels relative to a vessel equipped with such a radar system. Land based versions of such radar systems are also used to track vessels relative to a critical asset such as a nuclear power plant.
Terrorists, pirates, drug runners, smugglers, and vessels carrying illegal immigrants, however, have learned how to defeat attempts to monitor their activities by hiding in the “radar shadow” of larger ships and vessels. Using conventional radar systems, both a small craft at some distance from a critical asset (e.g., a ship or important installation) and a large vessel at some distance from the critical asset can be tracked. The closest point of approach (CPA) of the large ship to the critical asset is determined and the time to reach that closest point of approach (TCPA) is determined. No proximity alert is generated if the TCPA of the larger ship is greater than a predetermined threshold (e.g., 20 min) even if the CPA is small (e.g., 100 m). And, the CPA of the smaller craft may presently be too large to generate an alarm. But, suppose that the small craft has a vector (direction and speed) relative to the larger ship such that the smaller craft will intersect the larger ship. In just a few minutes, the smaller craft may enter the radar shadow of the larger ship and the smaller craft will then not be trackable by radar.
A typical radar system will then drop the track of the smaller craft because a radar update for it was not acquired for several minutes. The operator is not notified. 20 minutes later, when the larger ship is at its CPA relative to the critical asset with the smaller craft hiding in the radar shadow behind the larger ship, the smaller craft may maneuver at high speed towards the critical asset. At 30 knots, the small craft will reach the critical asset quickly. At best, a conventional radar system will have reacquired the track of the smaller craft for only about 4 seconds when its range to the critical asset is 60 meters or less. In many instances, this is not enough time to properly react to the smaller craft. Moreover, acquisition of a track at very short range is not as simple as at longer ranges because of receiver blanking after transmitter trigger, antenna elevation pattern losses, and steep angles into sea clutter. Also, many engagement systems are not designed to operate at very short ranges.
Conventional collision avoidance technology is able to determine if either the large ship or the small craft are on a collision course with a vessel equipped with the appropriate radar system. But, such collision avoidance technology cannot be used to determine if the small craft is maneuvering to hide in the radar shadow of the larger ship in order to approach the vessel with hostile intentions.